Aeronautical communication systems are frequently based on a satellite link, between a satellite and an aircraft, because, when it is moving, the aircraft is continuously visible to the satellite, which is not the case with terrestrial stations. Satellite links likewise allow a longer range than terrestrial links.
In the vicinity of some strategic sites, such as urban areas or airports, however, the aircraft will be in range of a terrestrial station.
The latency of the satellite link, connected with the propagation time from or to the satellites, is greater than that of terrestrial links. In order to limit the impact of this latency on the overall performance of the communication, it is necessary to implement a reliable data link, so as to have as little recourse as possible to processes for retransmitting the signal, such as ARQ or HARQ (standing for Automatic Repeat reQuest/Hybrid ARQ denoting an error checking method for the transmission of data) processes.
The aeronautical propagation channel is subject to many phenomena degrading the quality of the data link, such as, in particular, the Doppler effect, which are related to the movements of the aircraft.
The antennas used on a moving aircraft are generally antennas with an omnidirectional tendency in order to dispense with antenna pointing mechanisms and to simplify the implementation of antennas on the aircraft. However, for this type of antenna, the problem of multiple paths from reflections on the ground or on the fuselage arises, which is the origin of interference between received symbols.
Thus, depending on the geographical position of the aircraft and the movements of the latter, operation of the satellite data link can be greatly degraded, giving rise to systematic recourse to processes for retransmitting the signal.
The propagation channel between the aircraft and the terrestrial station is itself also subject to these constraints, but it is different than the propagation channel between the aircraft and the satellite station. Since the terrestrial station is closer to the aircraft and less constrained in terms of transmission power, the link budget of the terrestrial link will be more advantageous than that of the satellite link.